About 3.5 m long, says the UC Museum of Paleontology. Beside a person 1.75 m tall; ticks every metre.Body shape drawn for this site
- When
- Early Permian, about 299 to 273 million years ago
- Where
- Mostly Texas and Oklahoma, USA
- Size
- About 3.5 m long, says the UC Museum of Paleontology
Dimetrodon is often mistaken for a dinosaur, even in toy boxes. It was not one, and it was not a reptile either. It was a synapsid, from the same side of the family tree as mammals, and it died out long before the first dinosaurs.
It was the top predator of the Early Permian, about 3.5 metres long. Bony spines on its back rose nearly a metre and held up a sail of skin full of blood vessels. Its job is still argued over. The most popular idea is that it helped Dimetrodon warm up or cool down; the sail’s area keeps step with the size of the body, as you would expect if it moved heat.
Read more in Before the dinosaurs.
Also in mammals and their kin
- Brasilodon Possibly the oldest mammal, or one of its closest relatives
- Morganucodon A tiny early mammal named after Glamorgan
- Pakicetus One of the first whales, with four legs
- Dorudon An early whale that lived wholly in the sea
- Blue whale The largest animal ever to live
- Woolly mammoth An Ice Age elephant of the far north
Sources
- University of California Museum of Paleontology, Introduction to the Pelycosaurs
- Field Museum, Chicago, Dimetrodon is not a dinosaur
- Field Museum, Chicago, The fossil non-mammalian synapsid collection
- International Commission on Stratigraphy, International Chronostratigraphic Chart, v2026/06